Composite materials of plastics, especially rigid, semi-rigid, and flexible polyisocyanate foams in conjunction with metals and eminently suitable for the mass production of lightweight furniture, sporting goods, venicle bodies, construction, heat-insulated containers, or tubes industries. They may be produced, for example, by introducing the metal component into a mold and subsequently pouring the foamable reaction mixture into the mold, where it foams and hardens, preferably in an extremely dense form. This heavy densification, which is generally accompanied by the development of a cellular core and a solid outer layer, is obtained by measuring the quantity of reaction mixture introduced into the mold in such a way that, the foam would occupy a greater volume than the volume of the mold. The result of this is that the foam completely fills the mold and exactly reproduces the inner surfaces thereof. At the same time, the sheet of metal introduced is bonded to the foam.
Another method of producing composite materials of the type in question is to coat metal surfaces with a reaction mixture which reacts to form the plastic, preferably a foamed plastic. The strength of the bond between metal and plastic which is obtained in this way is inadequate for numerous technical applications.
In general, composite materials of metals and foams, especially polyurethane foams, are subjected to heavy mechanical stressing. These materials are only able to withstand this stressing if a permanent, shear-proof bond has been established between metal and plastic. In addition, the reinforcing effect of metals coated with polyurethane foams is only attained if the strength of the bond between polyurethane and metal is adequate. For this reason, several techniques have been developed with a view to improving the adhesion of polyurethanes to metals. For example, the metals have been sand-blasted before foaming (Piechota/Rohr: Integralschaumstoffe, Hansa Verlag, Munich/Vienna 1975) in order to obtain additional anchorage of the foam to the metal by roughening the surface. A variety of different adhesives have also been used (Piechota/Rohr: Integralschaumstoffe, Hansa Verlag, Munich/Vienna 1975) and, in some cases, have produced a certain improvement in the strength of the bond between the two materials. However, it has been found in practice that all these bonds gradually deteriorate and, in some cases, are completely destroyed by the effects of hydrolysis.
Bonds of high strength may be obtained, for example, by treating the metals before foaming with certain alkoxy silanes (Dow Corning, Pamphlet 03-023.03, July 1969; E. P. Plueddemann, Journal of Paint Technol., Nov. 1970). Unfortunately, the bond obtained by this method is not sufficiently resistant to boiling.